Molded plastic articles may be comprised of thermoplastic or thermosetting materials. Thermosetting materials are irreversibly chemically transformed from an initial granular state into a solid by heating in a mold. Thermoplastic materials are melted from an initial solid base and are resolidified by cooling in a mold. The mold shape is defined between open cavities formed in opposed mold plates or dies. Molding processes include compression molding and injection molding, among others.
Compression molding is a manufacturing process which can be described generally as one whereby a molded plastic article is formed from a thermosetting material. The thermosetting material is placed into an open mold which is then closed and subjected to heat and pressure. The raw material is cured under these controlled conditions to take the solid shape of the closed mold cavity. Heat may be applied by conduction, induction, or any conventional means, and pressure is applied by clamping opposed mold plates together. The finished articles are forcibly removed from the mold by ejector pins. The ejector pins may extend from an ejector plate which cooperates with a moving mold plate to push the casting out of the cavity from within when the mold plates are separated, or may be operated by a hydraulic ram or other known means coordinated with opening of the mold. Ejector pins normally leave blemishes on the product surface at the point of contact.
Injection molding is a manufacturing process whereby a thermosetting or a thermoplastic material is formed into the shape of a mold. Generally, the material is first formed into a fluid phase and is then injected under pressure into the mold. Thermosetting materials are cured to a solid phase under heat and pressure, thermoplastics are cured to a solid phase by cooling. Cooling may be accomplished by means of cooling lines extending through or around the mold.
A disadvantage common to these and other processes for manufacture of plastic castings is the expense associated with fabricating the mold plates or dies. These items must be of high quality steel in order to resist deformation throughout repeated cycles of curing temperatures and pressures. Furthermore, they are usually finely polished to produce a smooth product surface. Most importantly, the plates or dies must enable the fluid phase plastic to flow into and fill all voids in the mold cavity before it cures to a solid form. For this reason a main inlet conduit or sprue usually feeds a distribution system of runners terminating in gates through which the melt enters the mold cavity. A complex product shape may require a mold cavity to be formed with separate sections having differing outlines, depth, and surface area. Such mold cavities require a correspondingly complex arrangement of runners and gates. A deep cavity also presents a greater surface area where the cured material may stick to the cavity surface, and the mold plate must then be designed with the deep cavities more closely adapted to an ejector means. The expense of mold plates and dies can become very high whereby only a very large production run of products can be justified for each design shape.
This disadvantage of known molding methods and equipment becomes more pronounced when a series of products are to be manufactured having portions of related but differing shapes. Such products require individual plates to be fabricated for each design variation in the series. As noted above, mold plates are precisely fabricated components and must accommodate passages for cooling lines, heating lines, and other elements particular to the type of molding unit employed. Fabrication of numerous individual plates is accordingly very expensive.
Additionally, all mold plate pairs are slidably mounted on a heavy rigid frame to allow the mold to be opened and closed. For this reason changeover of mold plates is a labor intensive process causing substantial down time at the mold unit. Fabrication costs, labor costs and lost time are increased with each variation in product design shape.
The prior art is thus seen to fail to provide a cost efficient method and apparatus for the manufacture of both large and small production runs of plastic articles having related but differing design shapes.